Differential analog circuits have many advantages over single-ended designs that make them desirable to include in electronic systems. In particular, differential circuits have larger dynamic range, better common-mode and power supply rejection, and suppression of even-ordered distortion products. In most analog and radio frequency (RF) circuitry signals are single-ended at the board level because of the difficulty in matching components and the requirement of doubling the number of components to implement a differential circuit. Therefore, analog and RF applications typically perform a single-ended to differential conversion at the input of the system's integrated circuits.
In low-level RF applications, such as cellular phone, 2-way radios, or satellite radio receivers, the signals presented at the input of the radio receiver are very low-level signals, requiring that the single-ended to differential conversion be performed with high linearity and little noise added.
FIG. 1 shows a prior art method of performing a single-ended to differential conversion. As can be seen in FIG. 1, the RF source is input into the gate of transistor M1 and at termination resistor R.sub.in (typically 50 ohms), in order to match the RF source impedance, as will be appreciated by those skilled in the art. The control signal at the gate of transistor M2 is held at a fixed voltage to provide the same DC current through each of transistors M1 and M2. At lower frequencies, the input signal will vary the current through transistor M1 such that, due to the current source, an equal but opposite current is introduced into transistor M2. The converter of FIG. 1 runs into problems because the stray capacitance on the source of transistor M2 appears as a ground at RF frequencies, preventing current variations in transistor M1 from translating into current variations in transistor M2. (Especially when the capacitive impedance becomes significant with respect to the transconductance of transistor M2). In addition, the prior art converter of FIG. 1 introduces a significant amount of noise on the output. Noise is introduced from the RF source itself, from R.sub.in, and from transistor M1 and transistor M2.
The figure of merit most often used to characterize noise in RF circuits is the noise figure (NF). The noise figure of a circuit is defined as the decrease in signal-to-noise ratio (SNR) as a signal passes through the circuit. An ideal circuit has a noise figure of unity, or 0 decibels (dB). Its output noise will be the noise of the source resistance multiplied by the circuit's gain. Thus, the addition of the termination resistor adds 3 dB of noise, and with the input signal apportioned across two transistors, both contribute additional noise. Therefore, with the substantial noise and high frequency balance problems of the prior art converter of FIG. 1, this converter has limited use at high frequencies.
FIG. 2 shows another prior art converter to a differential amplifier. The transformer attached to the inputs of the converter performs the single-ended to differential conversion and applies the differential signal to sources of transistors M1 and M2. This produces a current differential output. Although the termination resistor has been eliminated in this design, the transformer itself has introduced a signal loss that effectively lowers the SNR. Therefore, this design also has a less desirable noise figure. In addition, the transformer is typically an expensive component and will limit the operating frequency of the design such that the converter is limited to the design range of the transformer. The transformer is also physically large, consuming printed circuit board space which is decreasingly available in cellular telephone and radio applications.
FIG. 3 shows yet another single-ended to differential converter disclosed by Durec, et al. in U.S. Pat. No. 5,497,123, incorporated herein by reference. As disclosed in that patent, the advantage of the circuit of FIG. 3 is that third-order non-linearities can be canceled by appropriate selection of components and bias currents. Unfortunately, the circuit was not designed to be used in low-voltage applications and does not have an optimized noise figure. As will be appreciated, the stack of devices Q1, R1, R2, and Q3 each introduce direct current (DC) voltage loss across each device, requiring a minimum of voltage differential to be applied across those devices. Therefore, with systems operating at 5 volt power supplies, the voltage loss across these devices is insignificant, but as operating voltages are lowered to 2.7 or 1.8 volts, the circuit of FIG. 3 becomes impractical because of the voltage loss across devices. The noise figure of this converter is also not optimized. Because of the balancing of components to produce the cancellation of third-order non-linearities, Durec, et al. did not optimize the noise figure of their configuration. As will be appreciated, each of the transistors Q1, Q2, and Q3, each of the resistors R1 and R2, and the RF source each introduce noise at the outputs.
What is needed, then, is a single-ended to differential converter which has a low noise figure and which is also small and inexpensive. Such a converter is provided by the present invention, whose features and advantages will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings.